The electromechanical brakes of an aircraft braking system conventionally comprise a stack of friction disks and a plurality of electromechanical actuators, each of which has a pusher suitable for being moved by means of an electric motor in order to selectively apply a pressing force on the stack of disks. The actuators also include a blocking member for blocking the pusher in position.
Such electromechanical brakes are generally designed to operate in three braking modes, referred to herein as “controlled” mode, “parking” mode, and “protection” mode.
In controlled mode, which is activated as a result of a pilot of the aircraft actuating brake pedals or an automatic braking lever, braking control means associated with a brake receive a controlled braking setpoint and they deliver electrical power to the electric motor of each actuator of the brake so that the actuator generates a pressing force corresponding to the setpoint. By way of example, the control means comprise an electromechanical actuator controller (EMAC) and an electronic braking control unit (EBCU).
In parking mode, which is activated as a result of the pilot actuating a parking selector, the control means deliver electrical power to the electric motor so as to bring the pusher into a position in which it exerts a certain pressing force, and then they activate the blocking member to block the pusher, and they deactivate the motor. Parking mode is used during stages of parking for the purpose of keeping the aircraft stationary when it needs to be prevented from moving on the ground for a certain period of time. Given the variations in the dimensions of the components of the brake (heat sink, torque tube, plate for holding the heat sink, etc.) as a function of temperature, the force to be maintained varies over time and it is necessary to provide for the control means to implement adjustments in order to adjust the position of the pusher.
Protection mode makes it possible to protect the electric motors of the actuators by switching off their power supply in order to prevent their temperature from becoming too great. The blocking members of the actuators are then activated and act on their own to maintain the looked-for pressing force. Protection mode is generally activated as a result of a command that is internal to the braking system, in a manner that is transparent for the pilot.
In parking and protection modes, the pressing force is referred to as a “holding” force, i.e. a pressing force that is maintained solely by the action of the blocking member on the pusher.
It is found that the parking and protection modes can be used in situations that apply high levels of stress on the brakes.
It can thus happen that parking mode is used to prevent an aircraft from moving while carrying out thrust tests on propulsion engines. The holding force that the blocking members of the actuators need to be able to maintain is then very high.
When a pilot uses controlled mode to hold an aircraft stationary at a runway stop point, the aircraft having its propulsion engines in action, it is possible that the temperature of the electric motors of the actuators becomes too high as a result of delivering a maintaining force for a long waiting period. Protection mode is then activated by the braking system, thus requiring blocking members to be dimensioned so as to be capable of maintaining this holding force, which can be large.
Such uses of a blocking member in parking and protection modes presents two technical difficulties that are frequently encountered by the designers of braking systems or of brake actuators.
The first difficulty concerns the logic used for making adjustments when the aircraft is in a parking stage. This logic involves causing the position of the pusher to be adjusted. When the way in which the holding force varies is not known, adjustments are performed by applying high forces, thereby tending to age the actuators prematurely.
In addition, the blocking members are often dimensioned as a function of the pressing force needed for holding the aircraft stationary at a runway stop point or when performing thrust tests. Since these situations require a holding force that is much greater than that needed during a parking stage, the blocking members are overdimensioned considerably relative to the usual needs during a parking stage.